PINEAL EYE IN LACERTILIA. 221 



Taking thus the animal kingdom as a whole, we see that the 

 epiphysis presents in all forms below mammals the following 

 two points in common with regard to its structure. 



(1) It originates as a hollow vesicular outgrowth stretching 

 forward from the roof of the thalamencephalon. 



(2) It becomes divided during development into two main 

 divisions. 



(a) A distal vesicle. 



(b) A stalk (hollow or solid) connecting {a) with the 

 brain roof. 



In Mammalia the first of these two points obtains (except 

 that the structure stretches backwards instead of forwards), 

 but degeneration of the tissues sets in at an early period and 

 secretion of solid material takes place in the part corresponding 

 to the hollow vesicle of other forms. 



In Aves both points obtain, but in course of development 

 the distal vesicle becomes solid and highly vascular. Below 

 Aves it apparently remains vesicular throughout life save in 

 the Anura where the distal division separates ofl", becomes 

 solid, and lies extracranially. 



So far as is known to us at present the distal portion becomes 

 most highly modified in Lacertilia ; further investigations into 

 its structure in other groups is needed, but, as far as our present 

 knowledge goes, we are justified in saying that in Lacertilia 

 alone, amongst living forms, the distal part of the epiphysis is 

 modified into an eye and the tissues between it and the external 

 surface are modified so as to allow of the easy transmission of 

 rays of light to the organ. 



In Petromyzon certainly the structure of the organ as 

 figured by Ahlborn resembles somewhat an eye, but closer ex- 

 amination reveals considerable difierences between it and the 

 eye of any Lacertilian. 



(1) Its division into two vesicles, one above the other, is a 

 point of some importance, indicating that in this case develop- 

 ment takes place along another line from that pursued in 

 Lacertilia, where the vesicle always remains single. 



(2) The absence of true retinal elements or lens is remark- 



